Would you make an honest biopic about Stalin today without any mention of the starvation of the kulaks, the horrors of the Gulags, the persecution of intellectuals, Holodomor or the Great Purge in the thirties?
Churchill might not quite reach Stalin's level of cruelty and despotism, but he's not far behind. Stalin was a tyrant in his own country and the eastern block, while Churchill was a tyrant in the British colonies, but also in independent states on their way to become democracies until Churchill did what he could to put an end to it.
The Bengal famine in 1943 was for sure the Darkest Hour for the 3 million Indians that were sacrificed. At the beginning of 1943 the food supply began reach acute levels in the Bengal, and refugees began to flow into the cities (mainly Dehli and Calcutta). Despite appeals from various organizations and local officials, the authorities in Delhi and Calcutta on the orders of the British War Cabinet, refused to declare the area as a famine zone in accordance with the Famine codes established at the end of the 19th century.
The reason for this was so as to not disturb the war efforts in Europe, which was the first priority of the British Government. Despite early warning about the famine and despite the fact that massive amounts of grain on its way to Europe could easily be redirected back to India without endangering the war effort, Churchill refused to help the Indians.
The disastrous Gallipoli Campaign, the sacrifice of the Scottish 51st Division in 1940 or the plan to attack and possibly nuke the Soviet Union in 1945 isn't mentioned either. Neither is his colonial blood lust: The advocacy of using chemical weapons against Iraqi Kurds in 1919-1920; the creation of the notorious paramilitary group Black and Tans which imposed a reign of terror on Ireland and Catholic civilians in 1919-1922; the brutal putting down of the uprising in Kenya, with concentration camps, rapes, torture and massacres in 1952-1960; his role in the death of 28,000 Boers in concentration camps in South Africa during the Second Boer War; his role in the firebombing of Dresden in 1945, a civilian town of no military significance, resulting in up to 300,000 civilian casualties. And last but not least the coup against Mossadeq and Iran's democracy in 1953.
If your argument is "he helped save Europe from he evil Nazis", consider that he was responsible for the creation of a network of concentration camps and the death of millions of people of his own.
With all this in mind, it's hard to stomach all the glory, unreserved homage and pomp and circumstance that is Darkest Hour, however impressive Gary Oldman's portrayal is.